1. Field
The subject invention relates to optical systems for examination of specimen and, more particularly, to such system that required cooling of the specimen, such as semiconductor chips.
2. Related Art
Various optical systems use objective lens to observe specimen. The objective lens may be tailored to provide a given magnification and field of view. Generally lower magnification provides larger field of view. Therefore, in some implementations several objective lenses are positioned on a turret so that different magnifications can be selected by the user. For example, a user may first select low magnification with high filed of view so as to locate a feature of interest on the specimen, and then select a higher magnification lens to examine the feature up close. For increased numerical aperture, it is also known to use a solid immersion lens (SIL) in conjunction with the objective. The arrangement of objective lens and a SIL can be referred to a collection optics. Such collection optics is particularly beneficial for observing specimen at high magnification, for example, for inspection and testing of semiconductor chips, as described in, for example, U.S. Pat. Nos. 6,594,086; 6,621,275 and 6,828,811. Examples of commercial systems utilizing such optics include laser voltage probing (LVP) such as the Ruby® and emission microscopy, such as the EmiScope® and Meridian®, both available from DCG Systems of Fremont, Calif.
When using such optics for inspection of chips, the encapsulation of the chip is removed, and the chip's substrate thinned, sometimes to 100 microns or so. The chip may also be stimulated by test vectors, e.g., using a conventional Automated Testing Equipment (ATE). Under such conditions, the chip tends to overheat, or at least operate at temperatures above its normal operating temperatures. Therefore, it has been suggested to use fluid spray to cool the chip. This is disclosed in, for example, U.S. Pat. Nos. 6,621,275; 6,836,131 and 7,102,374.
However, a problem exists in the art that sometimes it is desirable to switch the magnification during inspection of the chip. Since the chip receives test vectors, it generates heat and needs to be constantly cooled. Consequently, if the cooling is stopped in order to switch magnification, the testing would also have to be stopped so as not to cause the chip to overheat. What is needed in the art, therefore, is a collection optics having variable magnification which enables switching of objective lens without having to turn off the fluid cooling.